Abstract

Neptune and Uranus are observed with Keck II NIRSPEC in an attempt to detect H+(3) emission from Neptune. In this set of observations, H+(3) emission remains undetected at Neptune, whereas line-resolved emission from Uranus was observed with a signal-to-noise ratio of similar to 100. Using this, we have derived an upper limit of the column-integrated H+(3) density on Neptune of 1.5 (+4.8(-0.9))x 1013 m-2, assuming a temperature of 550 +/- 100 K. This value improves the previous established limit by a factor of 20 and shows that the H+(3) density predicted by the best available model overestimate the density by at least a factor of 3.In addition, the solar reflection continuum of Neptune in the K and L' bands is seen to be brighter on the Northern hemisphere by a factor of similar to 2, whereas previous observations had noted the solar reflection as being brighter on the Southern hemisphere.